This invention relates to manufacturing an article by powder metallurgy comprising compacting and sintering a mixture of iron powder and hypereutectic copper phosphide powder. More particularly, this invention relates to sintering a green compact comprising iron powder and copper phosphide powder at an relatively low temperature to produce an iron-base article having improved mechanical properties.
In the powder metallurgy art, iron articles are typically produced by compacting iron powder and sintering the green compact by heating in a furnace at a temperature between about 1120.degree. C. to 1200.degree. C. for approximately one-half hour. Under these conditions, it is estimated that sintering utilizes up to about 70% of the total energy required to manufacture the article. It is also estimated that a decrease of as little as 100.degree. C. in the sintering temperature would reduce energy usage more than 20% and also lower the cost of furnace maintenance by as much as half. Despite this economic incentive, the high sintering temperatures are required to produce articles having acceptable mechanical properties, particularly strength and ductility.
In addition to the sintering temperature, the mechanical properties are also substantially affected by the composition of the green compact. Plain iron powder alone produces a sintered article having good ductility, but limited strength. One method for improving product strength comprises admixing a powder containing an effective alloying agent with the iron powder prior to compacting and sintering. For example, small amounts of iron phosphide Fe.sub.3 P powder are commonly added to plain iron powder. During sintering, the phosphorus diffuses throughout the iron compact, forming a hardened ferrite alloy and thereby strengthening the product. However, sintering temperatures of 1100.degree. C. or higher are necessary to achieve significant strengthening within reasonable furnace times on the order of one-half hour. Even a small decrease in the sintering temperature results in a substantial reduction in these mechanical properties. Thus, the phosphorus-alloying sintering temperatures are comparable to those of iron alone. In addition, iron compositions containing phosphorus or other strengthening agents exhibit significantly reduced ductility in comparison to plain iron. Thus, the powder metallurgical art has long sought agents for admixing with plain iron powder to produce articles having an optimum combination of mechanical properties, that is, acceptably high strength and acceptably high ductility.
Therefore, it is an object of this invention to provide a powder metallurgical method for forming an iron article by mixing plain iron powder and a minor portion of an additive powder, which additive powder is a hypereutectic copper phosphorus composition, compacting the mixture and sintering to produce the article having improved mechanical properties, particularly in view of the sintering temperature. The product article features phosphorus distributed in the iron grains to improve strength and a copper-rich residue coating the pore surfaces, which also enhances product strength and ductility.
It is an object of one aspect of this invention to provide a method for forming an iron article by compacting a plain iron powder mixture containing a minor portion of a powder having a hypereutectic copper phosphorus composition and sintering the compact at an relatively lower temperature to diffuse phosphorus into the iron and form a copper-rich residue on the pore surfaces. The product article exhibits an acceptable combination of ductility and strength, comparable to conventional powdered iron articles sintered at significantly higher temperatures. More particularly, the method provides improved strength, such as conventionally achieved by iron phosphide addition, but at a significantly lower temperature.
In another aspect, it is an object of this invention to provide a method for forming an improved iron article by compacting a mixture comprising a major portion of plain iron powder and a minor portion of a hypereutectic copper phosphorus powder, and sintering. The product article exhibits improved mechanical properties, particularly strength, in comparison to conventional powdered iron articles sintered at equivalent temperatures, such as a plain iron product or a comparable iron phosphorus derived product.